Process of curing friction material



July 1e, 1929. E. R. 'R'oss- 1,721,135

PROCESS OF CURING FRICTION MATERIAL Filed March 12, 1927 (hmmm, s

Patented July 1s, 1929.

UNITED STATES PATENT oPFlcE.

ELMEB R.`ROSS, OF BRIDGEPORT, CONNECTICUT, ASSIGNOP. T THE RAYBESTOS COM- PANY, OF IBRIDGEIPORT,` (,JNNECTICUT, A CORPORATION OF CONNECTICUT.

raocnss or GURING FRIcrIoN MATERIAL.

Application led March 12, 1927. Serial No. 174997.

This invention relates to a new and improved process of hardening a binder material in a fibrous stock, and more especially for the economical and efficient manufac- 5 ture of friction materials having a fibrous baseJ ofasbestos or including asbestos, such for example as brake linings or clutch facings.

It 'is an object of the invention to produce a method for treating these materials, especially in curing or hardening the binder, which will give a better and a more uniform product than is now generally secured in the methods now employed, and also a vmethod in which the time required for the v curing operation is reduced over the methods now generally in use.

In the manufacture of friction materials or elements, and particularly friction materials having a woven asbestos yarn fabric base, it is customary to impregnate the fabric base with liquid binder substances, such as drying oils, asphalti'c materials, pitches, heat setting synthetic resins and other adhesives or hardening substances, or substances catalyzin-g a hardening reaction, or solutions of the binders, and subsequently subject the impregnated fabric to air heat, such as in a furnace or oven, in order to harden the binder.A For convenience I will call this method the open curingl method. In some binders al certain amount of oxidation is a desirable factor or may be an essential factor in the setting or hardening of the binder, such, for example, as the vegetable drying oils, but in the majority of other binders at present in use for friction materials the hardening of the primary satura-nt consists in the distillation or evaporation of the more volatile portions or the constituents having the higher vapor pressures at the curing temperatures, such as solvents contained therein, or some internal chemical change which induces hardening,

.45 or there may be a combination of both ef-` fects. This invention is especially advantageous for use in conditions where oxidation is a small, negligible or even an undesirable factor in the hardening of the binder, but' it is also very advantageous for a preliminary or initial curing or treatment in the hardening of certain binders where some oxidation is desired, as will be more fully discussed later.

As an example, it is known in the manufacture of brake linings to saturate tape made, from a yarn consisting of asbestos fibre, cotton fibre and fine brass wire in fa solution of gilsonitel or other hard petroleum residue carried in a gas oil solvent, the latter selected for economy of priceover other light solvents, such as benzol or gasoline. The saturated asbestos yarn tape is then heat treated in an oven in an oxidizing atmosphere having temperatures between about 350o F. and 450 F., whereupon the lower boiling gas oil distills away leaving the dry hard pitch as a residual binder. the solvent used and the thickness of the stock, the time necessary for drying off the solvent may range from thirty minutes to several hours. It is noted vthat the higher Depending on the temperature,

the temperature and the shorter the time at that temperature in this open curing method the greater is the difference in hardness between the surface and the intesity in commercial asbestos yarn spinning henomenon, 'at thel higher and weaving, is very rapidly attacked by 4air heated above 350 'F. decreasing the strength and pliability of the resultantbrake lining. The case hardening of the film on the lining also reduces thev pliability of the lining. Four. serious disadvantages of air curing' at elevated temperatures are,

therefore, expansion of the saturant audits resultant gathering at the surface forinin a film, case hardeningof this film, rapti'il distillation and surface oxidation of t 's film which decreases the uniformity' of curing of the fabric, and partial or complete destruction of the cotton fibre. Also, as

is well known,igases are very poor'conductors and transmittersfof heat, the heat conductivity of air, for example, being of the order of .000057 according to Winkelmann.

4The present invention is designed to provide thi'ee factors which are extremely advantageous for rapid uniform curing and hardening of a bindermaterial Within a 'fibrous base, namely first, exclusion of' air or an oxidizing atmosphere, second, removal of excess saturant, and third, rapid conduction of heat to the material. vPrimarily the.

heat setting ofv the binder thus take place with complete exclusion of' air or an oxidizing atmosphere, and because of the fact that liquids have a much greater thermal conduc- V tivity than gases, ranging from about ten times in organic liquids to about one hundred thousand times in molten metals, the required heat is much more .rapidly supplied to the article being treated and there is thus much more rapidpenetration ofthe heat into the article. Thus for some purposes We have used a molten type metalalloy with a heat conductivity of about .11, which is about twenty thousand times as great as that of air.

As a specific example we may saturate brake lining tape made of asbestos covered wire yarn in a 30% solution of gilsonite in gas oil at 250 F. to complete saturation which may be about 40% of this saturant by Weight. This may be led rapidly through a bath of type metal held at a temperature of 600 F. so that the total immersion time is about three minutes. The saturant expands rapidly under the high temperature and about 20% of the saturant is released from the poresof the tape by this expansion and floats to the surface of the metal. The residual saturant loses its gas oil solvent by distillation, the vapors also passing to the surface and out. The 'brake lining'onexit from the bath is quickly cooled in 'an atmosphere. of steam so as to prevent spontaneous combustion. After this treatment the gil sonite residue is left thoroughly distributed in the brake lining in a substantially dry condition, and the brake lining after rolling to size is then ready for use.A

It should be noted, as shown'by the above example, that the present method allows the use of'very much higher temperatures for distillation and curing than are safe for use Where air or gases containing air is or are the heat transferring mediums, becausel of Amuch higher rateof distillation is possible and no detrimental surface oxidation is produced. In the open process cure we cannot go above 450 F. safely without danger of burning, of course, depending on the ina-- terial used, but this is with the materials ordinarily used, While with the present method we can use as high temperatures as we please so long as it is not sufliciently high to produce detrimental carbonization of the materials. and of course, with the heated liquid material in direct contact with the article being treated there is not only more i'apid transfer of heat to the article but there is unich more heat available at the surface of the article.

The specific method above described is used merely as an example and this method is not necessarily limited to use with asbestos bases, but it may be used for any inorganic or noneom'bustible fibrous materials, or it may be used with a combustible material with lower temperatures forthe molten bath. Even with the use of combustible fibrous bases welsecure a much improvedproduct with a greatly reduced time required for the curing or treatment because we ni'ay use higher temperatures than is possible with the open eure as the air is excluded, but the temperatures must be kept below the point at which the materials will carbonize to an undesirable extent. The temperature of. the metal bath should be sufficiently high so that droplets ofthe metal will not stick to the material treated.

In the accompanying drawing I have shown sm'ewhat diagrammatically an apparatus which may be used in this treatment. In this drawing,

Fig. 1 is a vertical section through a furnace and saturating tank, and

Fig. 2 is a transverse vertical section showing a portion only of the furnace.

The furnace as shown comprises a chamber 5 in which the tank 6 is located carrying the molten metal or other heated liquid 7. Below the tank is a gas or oil burner 8 directed into a combustion chamber 9, and between the burner and the bottom of the tank are suitable baffles 10 and 11 of fire brick having openings 12 and 13 respectively for passage of the products of combustion to theunder side of the tank where they pass outwardly and upwardly at the outer sides of the tank through the flues 14to the chamber 5. The tank is supported on suitable' supports'l.

At the opposite endsv of the tank are guide 'Outsideythe furnace is a tank 23 holding asuitable saturant 24, the tank having suitable guide rollers 25 to hold, the tape beneath the saturant. Scrapers 26 are provided at the outlet from the tank to scrape the surplus saturant from the tape as it leaves the tank. At the outlet end of the tank 6 carrying the molten metal or other heated liquid used is a seal 27 of granular material, such as powdered charcoal carried by a swinging support 28. A block 29 composed of a pipe closed at both ends secured to the lower cnd of this support will float in the metal and hold the support against the surface of the tape as it leaves the metal bath. A neutral or nonoxidizing atmosphere is maintained in t-he chamber 30 where the tape is cooled after leaving the molten'metal. This may be conveniently done by means of steam pipes 3l discharging into this chamber.

In operation the coil of the tape 32`is mounted adjacent the saturating tank 23 and passes through the saturant in this tank, between the Scrapers 26 and then through an opening 33 into the furnace where it passes over the drums 20 and 16 into the tank 6 where it passes through this tank under the molten metal or other heated liquid used. It then passes out the other end of this tank into they non-oxidizing atmosphere in the chamber 30 where it is rapidly cooled to a temperature below the temperature which would cause spontaneous combustion if the tape wereexposed to the air. It may then be wound up into a suitable coil, as shown It will, of course,'be understood that this 4is merely an examplev of how the process may be carried out and large variations in types of furnace maybe used as found desirable. It will also be understoodY that a number of different tapes may be passed through the furnace ,at the same time, asy

indicated in Fig. 2, and further that it is not necessary that the tape be Vsaturated in a tank 23 associated with the heatin furnace. In fact, `it is sometimes desira le to "saturate the tape and then let it stand a certain time before heat treating. It-will, of course, be understood that the time of immersion in the saturant and the time of immersion in the heated molten metal or other liquid may be controlled by varying the sizes of the tanks and varying the rate at which the tape lpasses through the tanks.

It should be understood that the basic idea of this process of hardening binders in fibrous or cellular materials is to heat the material within and by the medium of a heated liquid that does not interfere with the desired hardening, and'itis capable-of much variation and many uses that are not described here. As has been shown, any binder material that depends for its hardening onthe removal of its more volatile components, or on an internal chemical change resulting in hardening and which is induced or accelerated by the application. of heat, may be hardened within a molten metal.

However, there are cases where molten metal may be undesirable and in these cases molten salts, for example, boric acid or sodium sulphite, or waxes (paraiiin, montan', etc.) fats, oils o1' otherliquids may be used, the only requisite being that the liquid used for transmitting the heat shall not interfere with or prevent the desired hardening. Molten metals are preferred as the heating medium because they do'not wet or adhere to the .article being heated. If hot oils or some material which does not combine with or dissolve the saturant but which adheres to the article is used then, of course, a separate operation isrequired laterto wash it ofl" or dissolve it. For example, if we use as the binder a phenolic resin or a synthetic urea formaldehyde `resin Which is` incompatible with paraffin or parain oil we effect the curing under this paraffin or paraflin oil and on the removal from the curing desirable may be regulated by the length of the dip, that is, the length of the path through the molten material, the speed of movement of the material and the temperature of the bath.

It has been found that in open curing as above described there is a migration lcaused by expansion of the saturant which carries some of thefbinder out to the surface, 'so that 1n the open cure there is nota uniform distribution of the binder throughout the article at the end of, the curlng process, and there may be much less binder at the center of the artioleas the greater proportion of it has been carried tov the surface. This 1s partly due to the fact that in the open cure there is very sloW transfer of heat to the interiorl of the article and most of the curing appears to be on'the surface. In

the present method the rapid heating causes the saturantto expand against the resistance of the liquid bath forcing the saturant uniformly` and completely throughout the libres, also the more volatile constituents are driven off within the article as well as at the surface, leaving the heavier or bindin constituents substantially equally distributed ferring heat from the, molten metal bath to the center-fand throughout the element being treated. This method can, therefore, be used as an initial curing to prevent subsequent migration of the binder to the surface Where, as in certain types of binders or saturants, some oxidation is desired. With this type of binder the article is given a preliminary curing by this method to drive off the lighter constituents and remove excess of saturant which might later expand to the surface and form an undesirable film, and then it is given a cure in the open method to secure the oxidation desired. We can thus get a moreuniform curing and a much more uniform final stock even though some oxidation is desired or some open curing is used. This method gives toughness without brittleness in the finished product. In carrying out the method the melting point of the alloy should of course, be reg' ulated according to the temperature desired for'the curing. Metals with melting points from 350 F. ,to 625 F. or higher can be employed dependent upon the, treatment being used, while with different materials temperatures may be used ranging 'from Aabout 250 F. to a temperature which will not give undesirable carbonization of the stock used in the friction element, 250 F. being about the lowest temperature at which the v,curing can be effectively and practically carried out', although the curing.is ordinarily carried out at a much higher temperature.

Having thus set forth the nature of my invention, what I claim is:

1. A process for making a friction element having an asbestos fibre base which includes. treating the fibre with a primarily liquid binder, and thencuring by heatin -the element within and by the medium o a liquid substantially incompatible with the binder. at a temperature below that which would carbonize the base and sufliciently high to rapidly and uniformly cure and harden the binder.

2. A process for making a friction element which includes weaving' a yarn having an asbestos'fibre base into a tape, treating the tape with a normally liquid binder, and then curing by heating the tape within and by the medium :of a liquid substantially incompatible with the binder at a temperature below that which fould carbnize the yarn and sufliciently high to rapidl and uniform- -lyy cure and harden the bin er.

3. A process for making a friction element-having an asbestos fibre base which includes treating the fibre with aprimarily liquid binder and then curing by heating the element withinand by the medium of a molten metal at a temperature below that which would earbonize the base and sufficiently high to rapidly and uniformly cure and harden the binder.

4. Aprooess for making a friction ele-v ment which includes weaving a yarn having an asbestos fibre base into a tape, treating the tape with a normally liquid binder, and then curing by heating the tape within and by the medium of a molten metal at a tem-v perature below that which would carbonize the base and sufliciently high to rapidlyvand uniformly cure and harden the binder.

5. A processfor making a friction element having an asbestos fibre base which includes treating the fibre with a primarily liquid binder and then curing by heating the element within and by the medium of a liquid substantially incompatible with the binder at a temperature below that which would carbonize the base and sufiiciently high to rapidly and uniformly cure and harden the binder, and then exposing the clement while heated to an oxidizing atmosphere to oxidize some of the binder at the surface of the element.

6. A process of making a friction ele- 'ment having an asbestos fibre base which includes treating the fibre with a primarily 4uid substantially incompatible with the binder, and then oxidizing some of the binder by exposing the element to an oxidizing atmosphere.

8. A. process of making a friction element which includes weaving an asbestos covered wire into a tape, treating the tape'with a normally liquid binder, and then curing this l element by heating the element within and by. a molten metal.

9. A process ofmaking a fri'tionelement which includes Weaving an asbestos covered wire into a tape, treating the tap with a normally liquid binder, removing the surplus binder by heat-ing the element within and by the medium of a molten metal, and then curing in a heated oxidizing atmosphere.

10. The process of curing a friction element having a fibre base which includes. treating the fibre with a hardening binder,

`then heating the element within and bythe medium of a molten metal, and then heating ment with a normally liquid binder, and the element in cont-act with air to a suilithen curing by heating the element in and ciently high temperature to oxidize some of by the medium of a heated liquid substan- 10 the binder. tially incompatible with the binder.

5 11. A process of making a friction ele In ,testimonywhereof I alix my signamentv which includes weaving the element ture. 'from asbestos covered wire, treating the ele- ELMER R. ROSS. 

